An infectious parasite spread by cats may be a cause of brain cancer in humans, research suggests.

The single-celled organism Toxoplasma gondii infects about a third of the world’s population.

Often it causes no symptoms, but the parasite can be fatal to unborn babies and damage the nerve systems of people with weak immune systems.

The new study shows a positive correlation between rates of infection by T. gondii and brain cancer incidence around the world.

Scientists collected global data on brain cancers in men and women and compared them with figures on T. gondii prevalence.

Adjusting for a range of factors that can influence brain cancer statistics, the researchers found that cancer rates went up with greater exposure to the parasite.

Across the range of infection prevalence, from four per cent to 67 per cent of the population, T. gondii was associated with a 1.8-fold increase in the risk of brain cancer.

The scientists, led by Frederic Thomas, from the CNRS research institute in Montpellier, France, wrote in the Royal Society journal Biology Letters: “We feel our results are sufficiently strong to propose that T. gondii potentially increases the risk of brain cancer in humans.”

Previous research had already linked the parasite to brain tumours in animals.

There is also other evidence that T. gondii has effects on the brain leading to changes in behaviour.

Infected rats are known to lose their fear of cats, making it more likely that they will be killed and eaten. This is advantageous to the parasite, since it reproduces inside the intestines of cats. The organism’s egg-like oocysts are shed in cat faeces which may then contaminate food or infect other animals.

Some studies have suggested that T. gondii can alter the behaviour of humans too, making men more aggressive and even causing women to cheat on their husbands. Other research has pointed to a strong link between T. gondii and schizophrenia.

Chief causes of infection in humans are consuming undercooked meat, especially lamb, pork and venison, and ingesting water, soil or anything else contaminated by cat faeces.

The scientists added: “Clearly, further research is necessary to determine the proximate links between T. gondii and different types of brain tumours and to investigate a mechanism of action.

“Establishing a link between T. gondii and brain cancers could open the door to potential means to reduce cancer risk.”